Mutant FOXL2C134W highjacks SMAD4 and SMAD2/3 to drive adult granulosa cell tumors.

The mutant protein FOXL2C134W is expressed in at least 95% of adult-type ovarian granulosa cell tumors (AGCT) and is considered to be a driver of oncogenesis in this disease. However, the molecular mechanism by which FOXL2C134W contributes to tumorigenesis is not known. Here we show that mutant FOXL2C134W acquires the ability to bind SMAD4, forming a FOXL2C134W/SMAD4/SMAD2/3 complex that binds a novel hybrid DNA motif AGHCAHAA, unique to the FOXL2C134W mutant. This binding induced an enhancer-like chromatin state, leading to transcription of nearby genes, many of which are characteristic of epithelial-to-mesenchymal transition. FOXL2C134W also bound hybrid loci in primary AGCT. Ablation of SMAD4 or SMAD2/3 resulted in strong reduction of FOXL2C134W binding at hybrid sites and decreased expression of associated genes. Accordingly, inhibition of TGFβ mitigated the transcriptional effect of FOXL2C134W. Our results provide mechanistic insight into AGCT pathogenesis, identifying FOXL2C134W and its interaction with SMAD4 as potential therapeutic targets to this condition.